The present invention relates generally to a focus detection optical systems for digital single-lens reflex cameras or the like and an imaging apparatus incorporating the same, and more particularly to a focus detection optical system operable to divide the pupil of a taking lens into two pupil areas so that two secondary object images are formed by a light beam passing through each pupil area to detect a relative position relation between two such secondary object images thereby detecting the focus position of the taking lens, and an imaging apparatus incorporating the same.
So far, there is a phase contrast type focus detection optical system available, in which an image (primary image) formed by a taking lens is re-formed into two images on a light receptor element array via a pupil division optical system built up of a condenser lens, a pair of aperture stops and a pair of re-imaging lenses, and the light intensity distributions of two such images are compared for correlative operation to find a spacing between the two lenses, so that the defocus quantity of the primary image formed by the taking lens is obtained.
For instance, let Y0 be the spacing between the two images on the light receptor element array when the taking lens is in focus, and Y1 be the spacing between the two lenses when it is out of focus. The difference δ=Y1-Y0 is correlative to the defocus quantity of the primary image; if this difference δ is found, focus detection can then be implemented.
The first requirement for such a focus detection optical system is that the two images on the light receptor element array are well correlative to each other. Poor correlations between the two images would lead to a decline of correlative operation precision as their spacing is found, having adverse influences on range-finding precision.
Heretofore, for improvements in range-finding precision or for correction of aberrations, it has been proposed to tweak the re-imaging lenses in particular (for instance, see Patent Publications 1 to 5).
With the focus detection optical system set forth in Patent Publication 1, a re-imaging lens is configured in a plano-convex or double-convex shape and at least one of a condenser lens is configured in an aspheric shape for correction of distortion.
With the focus detection optical system set forth in Patent Publication 2, an auxiliary lens is added to just before a re-imaging lens so that there are a total of two lenses involved, and at least one of a condenser lens is configured in a rotationally oval surface shape for correction of distortion.
With the focus detection optical system set forth in Patent Publication 3, a re-imaging lens is configured in a double-convex shape with varying quantities of decentration of its entrance- and exit-sides surfaces, thereby correcting distortion.
With the focus detection optical system set forth in Patent Publication 4, a re-imaging lens is configured in a double-convex shape with the same quantity of decentration of its entrance- and exit-side surfaces, and the center of an aperture stop is decentered from the re-imaging lens.
And one surface of the re-imaging lens is configured in an aspheric surface shape for correction of field curvature.
With the focus detection optical system set forth in Patent Publication 5, one surface of a re-imaging lens is configured in a spherical surface shape and the other in an inclined surface shape having a prism action for correction of distortion and chromatic aberrations.
On the other hand, there is now a growing demand for a lot wider range capable of focus detection for a taking range and a lot more range-finding points. One possible approach to that is that as many such optical systems as range-finding points are provided as is the case with the prior art focus detection optical system shown in FIG. 19. The prior art of FIG. 19 comprises an imaging device having a light receiving plane adapted to receive an image formed by a taking lens (not shown). And a predetermined imaging plane 1 equivalent to the light receiving plane of the imaging device is formed through an optical path splitter such as a quick return mirror, a half-mirror or the like by reflection of light at an auxiliary mirror located on a side through the quick return mirror. In order from the predetermined imaging plane I toward a light receptor element array E, the prior art further comprises a field mask M placed near the predetermined imaging plane I and having a plurality of apertures corresponding a range-finding area on an optical axis L (equal to the optical axis of the taking lens) and an off-axis range-finding area, a condenser lens member L1 having condenser lenses corresponding to a plurality of range-finding areas, an aperture stop member S having a plurality of pairs of aperture stops in the vertical or horizontal direction, and an integral re-imaging lens member L2 having a plurality of pairs of re-imaging lenses lined up vertically or horizontally corresponding to the respective aperture stops.
However, such arrangement would give rise to an increase in the size of the focus detection optical system and an increase in the laid-out space.
For this reason, there has been a specific arrangement proposed for the purpose of making a sensible tradeoff between a wider range capable of focus detection or a lot more range-finding points and a reduction in the size of the optical system (for instance, see Patent Publications 6 and 7).
With the focus detection optical system set forth in Patent Publications 6 and 7, size reductions are achievable by shared use of the re-imaging lens.
[Patent Publication 1]
JP(A)60-32012
[Patent Publication 2]
JP(A)62-25715 (JP(B)8-12321)
[Patent Publication 3]
JP(A)62-69217 (JP(B)7-31300)
[Patent Publication 4]
JP(A)1-224714 (Patent No. 2586089)
[Patent Publication 5]
JP (A) 62-79407
[Patent Publication 6]
JP(A)2001-21797
[Patent Publication 7]
JP(A)2003-75718
However, the setting of magnification of the focus detection optical system is important, too, for achieving a lot wider range capable of focus detection and a decrease in the size of the focus detection optical system. The decline of magnification might go in favor of size reductions, but incurs a decline of focus detection precision. Because the decline of magnification places some limitations on the movement of secondary object images on the light receptor element array, and as the pixel pitch of the light receptor element array cannot be made fine, it causes a decline of the minimum defocus quantity capable of detection. Thus, the focus detection resolving power declines with a decline of focus detection precision. Further, parts are more sensitive to fabrication errors, assembling errors, and adjustment errors.
On the other hand, increased magnification gives rise to an increase in the size of the focus detection optical system.
As discussed above, to make the range capable of focus detection wider, render the focal detection optical system smaller and achieve good enough focus detection precision, it is necessary to set up the focus detection optical system while setting proper magnification.
Referring here to size reductions by shared use of a re-imaging lens, it is when all range-finding points are covered with a pair of re-imaging lenses alone, as can be seen from a reference example of FIG. 20, that the highest efficient is obtained.
The focus detection optical system shown in FIG. 20 comprises, in order from a predetermine imaging plane I equivalent to the light receiving plane of an imaging device adapted to receive an image formed by a taking lens (not shown) toward a light receptor element array E, a field mask M placed near the predetermined imaging plane I and having a single aperture able to cover all range-finding points, a condenser lens L1 located near that, an aperture stop member S having a pair of apertures in the vertical and horizontal directions, a re-imaging lens member L2 having a pair of re-imaging lenses in the vertical, and the horizontal direction corresponding to the respective apertures, and a light receiving member E having a plurality of light receptor element arrays comprising light receptor elements in rows in the direction that the re-imaging lenses in pair form are lined up.
And, a pair of secondary object images formed by the re-imaging lenses are received so that there is a light intensity distribution obtained, corresponding to light beams coming from the respective pupils through any arbitrary focus detection area.
Then, a relative position relation between a pair of secondary object images received on the aforesaid light receptor element array is detected thereby detecting the focus position of the taking lens.
Such arrangement has a simplified construction; however, a wider range capable of focus, detection with a pair of re-imaging lenses alone would render it difficult to keep the flatness of the images, viz., field curvature in good enough shape. Of the sagittal (S) and meridional (M) image planes, it is thus necessary to tweak the field curvature of the meridional image plane in alignment with the direction of the light receptor element array.
Referring to how field curvature is in good enough shape, Patent Publication 4 teaches that one surface of the re-imaging lens is configured in an aspheric shape thereby correcting field curvature; however, processing into the aspheric shape is not easy, and any practical use of the aspheric surface is difficult, mainly because there is much difficulty in estimating aspheric parts.
Patent Publication 3 teaches that the quantities of decentration of the entrance- and exit-side surfaces of the re-imaging lens are varied. This might work favorably for correction of field curvature, but works against decreasing the magnification (absolute value) of the focus detection optical system and achieving size reductions, because the quantity of decentration of the aperture stop and the quantity of decentration of the entrance-side surface of the re-imaging lens have the same sign whereas the quantity of decentration of the exit-side surface of the re-imaging lens has the opposite sign, rendering it difficult to make sure the refracting power of the exit-side surface. In short, as the radius of curvature of the exit-side surface of the re-imaging lens grows small, the lens is less and less likely to be in shape.
Patent Publication 2 teaches a focus detection optical system wherein the auxiliary lens is added to just before the re-imaging lens so that there are a total of two lenses involved: this might go in favor of correction of field curvature. However, more lenses are needed for the re-imaging lens, resulting in increased parts costs. Further, the main refracting power is shared by only the exit-side surface of the re-imaging lens; this works against decreasing the magnification (absolute value) and size reductions.
The invention has for its object to provide an imaging apparatus equipped with a focus detection optical system that has a simplified construction capable of covering a wide focus detection area without using more components.